The present invention relates to a cleaning solution based on gas-expanded liquids. More particularly, the present invention relates polar organic solvents whose solubility characteristics are modulated through the use of liquid carbon dioxide.
In the twenty first century, the recovery, recycle and reuse of post-consumer plastic containers is an active area of interest for both the public and private sectors. Each year millions of pounds of plastic are used to produce injection-molded containers for the aforementioned products. Disposal of these plastic containers in municipal landfills is undesirable as the plastic may be contaminated with environmental pollutants, such as pesticides or hydrocarbons. Such plastics include, but are not limited to, polyethylene, polyethylene terephthalate (PET) and polyvinyl chloride (PVC). In many communities, these plastic containers are separated from refuse by consumers and are collected by recycling centers to be reprocessed into new forms. Additionally, the disposal of these materials is a waste of a potential secondary raw material.
Certain classes of polymeric materials are presently recycled in commercial systems relying in great part on collection of polymeric containers from residential and commercial sites. This system is complicated in that associated wastes with differing sources of polymeric materials may not be amenable to a single format of treatment. Although local jurisdictions may require some level of cleaning of the polymeric containers, the original liquids or powders may be insufficiently removed from the polymer. These materials may vary from water, beverages, detergents, oils, alkaline cleaners, and highly toxic materials, including pesticides. In addition, the containers may contain labels that are applied by adhesives of different strengths, and the label stock itself may need to be treated by distinct processes. A single cleaning process has been unlikely to act on all polymeric containers, at least in part because of the deficiencies in the cleaning steps that fail to provide a sufficiently pure supply of polymer that would enable direct recycling.
In order to processes these post-consumer materials, cleaning of the plastic must render a product free of any initial use contaminants. This involves the removal of a variety of surface contaminants which may include motor oil, food or beverage products, pesticides, cosmetics and detergents. Additionally, the solvent must be able to swell and dissolve labeling adhesives that remain from the initial use of the plastics. Current water based cleaning systems are unable to economically process the full spectrum of these contaminants in an environmentally responsible manner.
The variety of contaminants that remain on the refuse plastic pose a particularly challenging problem for a facility that wishes to collect and clean plastics from all sources. In some cases, such as motor oil bottles that are collected at commercial facilities, the plastic may be obtained with a single class of contaminant. However, for plastic that is segregated from household or generic commercial refuse, the mixture of contaminants poses a challenge when selecting a solvent. In either case, it is desirable to have a simple solvent system that is capable of removing any contaminant that may be encountered as well as any labeling and adhesives. Broad-spectrum solvents, such as acetone and methylene chloride, provide a thermodynamic solution to this dilemma; yet, these solvents are not environmentally acceptable. Currently, much of the cleaning is done with water-based systems. While the use of surfactants can make this an effective process, large wastewater streams are produced which contain the contaminant species.
Recycling of motor oil containers is illustrative of the problem. Motor oil containers typically are high-density polyethylene (HDPE) which lends itself well to recycling if it is sufficiently clean. However, residual oil coating the interior surface of the “empty” motor oil containers constitutes a contaminant that prevents use of the containers as high grade plastics. Based upon measurement of samples of used motor oil containers, this residual oil coating appears to average 4.6 percent of the weight of the used plastic container and can represent as much as 20 percent of the container weight. Estimates are that over one billion one-quart plastic containers were filled with motor oils in the United States in 1993. If 4.6 percent by weight of those containers is motor oil, the one billion empty plastic containers represent approximately 160 million pounds of plastic and over 7 million pounds of motor oil that could be recovered for reuse if an appropriate separation method were available. However, because the motor oils have not been easily separated from the plastic containers, the vast majority of these containers are currently disposed of in landfills, leaking oils into the soil and groundwater, and occupying significant landfill volume.
Ideally, pure liquid or supercritical carbon dioxide would be the best choice for a sustainable solvent. However, the solvent power of CO2 is too poor to sufficiently dissolve many of the adhesives and contaminant species encountered. Thus a more polar solvent must be employed, and preferably an environmentally benign polar solvent to maintain the environmental benefits. However, such polar solvents ideal for this application have low volatilities, making them difficult to remove or evaporate from cleaned substrates.